The heads on a hard drive are intricate in their design and operation, and are typically quite delicate. While they are analogous to the needle on a record player, they actually operate without contacting the surface they fly over. They contain tiny wire coils used to read from and write to the magnetic surface of the platters. A drive typically contains one head per surface, with some of the largest drives containing 10 platters and 20 individual heads.

Damaged media causes damaged heads, which themselves cause more damaged media. It’s a vicious cycle that worsens over time, with older drives (with lower tolerances and lower recording density) typically handling it better than modern drives. The last stage of this tends to be the inability to read data from the platters, which manifests the drive being un-detectable and sometimes (but not always) clicking or making unusual sounds.

Stuck heads (also sometimes called “stiction”) are exceedingly rare without accompanying read write physical trauma, especially on modern drives. On traditional hard drives, the heads fly over the platters on a film of air called an “air bearing” without contact, so under normal operation there is no friction between the two to allow them to stick. The heads will not even deploy off of the ramp (or parking area) until the drive detects that the platters are spinning at a sufficient RPM to facilitate this air bearing.

PCB damage is most commonly the result of a power surge, caused by things like an incorrect power supply (to an external enclosure) or using the wrong modular PSU power cables. PCB damage tends to be on the lowest side of the price spectrum as it does not usually require uncommon tools to diagnose or repair. The PCB is studded with components that self-sacrifice to prevent damage to components that would complicate recovery.

Recoverability after deleting / formatting a drive is dependent on a number of factors. The technology used by the drive, the exact way in which data was deleted / formatted, the original file system, presence or absence of encryption, how much new data written to the drive, etc.. In most cases, the majority of data is recoverable.

File system corruption can have a number of causes, from interrupted delayed writes to malware to drive failure. The success of recovery is dependent on the cause of the corruption and what fragments of the file system are still present and interpretable. The ability to piect together file system fragments and extrapolate missing file system information is what sets software recovery tools apart from one another.

In most cases, the hard drive’s firmware resides partially or entirely on the platters in a special fault-tolerant region called the Service Area. Because this is on the platters, it is subject to damage in similar ways to other data on the drive. Most commonly, firmware damage is a symptom or bad sectors / media degradation in the exact wrong location. Luckily, this is typically an easy job for specialist hardware interface tools to deal with.

Viruses and malware can cause data loss in a number of different ways, including rendering the operating system inoperable, deleting files for the sake of vandalism, and encrypting personal data for ransom (ransomware). In the case of the former two, data recovery is a matter of interpreting remnants of the file system and / or excavating files without a file system.